The Conductivity Change That Results from Absorption of a Photon

The Conductivity Change That Results from Absorption of a Photon 

The human eye is amazingly sensitive. After being in the dark for a time, one can perceive continuous light that is so weak that an individual rod cell absorbs a photon, on the average, only once every 38 min. A flash of light is detectable if approximately six rods each absorb one photon. This means that absorption of a single photon by any one of the approximately 3 x 107 molecules of rhodopsin in a rod 

The electrophysiological response of a rod or cone to light involves a change in the permeability of the plasma membrane to cations. In the inner segment and basal parts of the cell, the plasma membrane contains a Na+-K+ pump, which uses ATP to move Na+ out of the cell and K+ in. (Recall our discussion of ion pumps in chapter 17.) K+ 

Na+ is pumped out of the inner segment solid arrows) and diffuses back into the cell through channels in the outer segment (dashed arrows). In rods of vertebrates, the Na+ channels are held open in the dark, and they close in the light. (In invertebrates, the channels open in the light.) 

The hyperpolarization of the rod and the response of the bipolar cell are graded effects. Absorption of a single 

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